This invention relates to the formation of olefins by thermal cracking of whole crude oil. More particularly, this invention relates to utilizing whole crude oil as a feedstock for an olefin production plant that employs a hydrocarbon cracking process such as steam cracking in a pyrolysis furnace.
Thermal cracking of hydrocarbons is a petrochemical process that is widely used to produce olefins such as ethylene, propylene, butenes, butadiene, and aromatics such as benzene, toluene, and xylenes.
Basically, a hydrocarbon feedstock such as naphtha, gas oil or other fractions of whole crude oil that are produced by distilling or otherwise fractionating whole crude oil, is mixed with steam which serves as a diluent to keep the hydrocarbon molecules separated. The steam/hydrocarbon mixture is preheated to from about 900xc2x0 F. to about 1,000xc2x0 F., then enters the reaction zone where it is very quickly heated to a severe hydrocarbon cracking temperature in the range of from about 1450xc2x0 F. to about 1550xc2x0 F.
This process is carried out in a pyrolysis furnace (steam cracker) at pressures in the reaction zone ranging from about 10 to about 30 psig. Pyrolysis furnaces have internally thereof a convection section and a radiant section. Preheating is accomplished in the convection section, while severe cracking occurs in the radiant section.
After severe cracking, the effluent from the pyrolysis furnace contains gaseous hydrocarbons of great variety, e.g., from one to thirty-five carbon atoms per molecule. These gaseous hydrocarbons can be saturated, monounsaturated, and polyunsaturated, and can be aliphatic and/or aromatic. The cracked gas also contains significant amounts of molecular hydrogen.
Thus, conventional steam cracking, as carried out in a commercial olefin production plant, employs a fraction of whole crude and totally vaporizes that fraction while thermally cracking same. The cracked product can contain, for example, about 1 weight percent (xe2x80x9cwt. %xe2x80x9d) molecular hydrogen, about 10 wt. % methane, about 25 wt. % ethylene, and about 17 wt. % propylene, all wt. % being based on the total weight of said product, with the remainder consisting mostly of other hydrocarbon molecules having from 4 to 35 carbon atoms per molecule. For more information on steam cracking see xe2x80x9cPyrolysis: Theory and Individual Practice by L. F. Albright et al., Academic Press, 1983.
The cracked product is then further processed in the olefin production plant to produce, as products of the plant, various separate individual streams of high purity such as hydrogen, ethylene, propylene, mixed hydrocarbons having four carbon atoms per molecule, and pyrolysis gasoline. Each separate individual stream aforesaid is a valuable commercial product in its own right. Thus, an olefin production plant currently takes a part (fraction) of a whole crude stream and generates a plurality of separate, valuable products therefrom.
The starting feedstock for a conventional olefin production plant, as described above, has been subjected to substantial, expensive processing before it reaches said plant. Normally, whole crude is distilled or otherwise fractionated into a plurality of parts (fractions) such as gasoline, kerosene, naphtha, gas oil (vacuum or atmospheric) and the like, including a high boiling residuum. Thereafter any of these fractions, other than the residuum, could be passed to an olefin production plant as the feedstock for that plant.
It would be desirable to be able to forego the capital and operating cost of a refinery distillation unit (whole crude processing unit) that processes crude oil to generate a crude oil fraction that serves as feedstock for conventional olefin producing plants.
However, the prior art teaches away from even hydrocarbon cuts (fractions) that have too broad a boiling range distribution. For example, see U.S. Pat. No. 5,817,226 to Lenglet.
In accordance with this invention there is provided a process for utilizing whole crude oil as the feedstock for an olefin producing plant with neither inadequate cracking of light fractions nor excessive cracking of heavy fractions.
Pursuant to this invention, whole crude oil is preheated, as in a conventional olefin plant, to produce a mixture of hydrocarbon vapor and liquid from the crude oil feedstock with little or no coke formation. The vaporous hydrocarbon is then separated from the liquid, and the vapor passed on to a severe cracking operation. The liquid hydrocarbon remaining is subjected to mild steam cracking at from about 800xc2x0 F. to about 1,300xc2x0 F. until it is essentially all vaporized and then passed on to the severe cracking operation. Any residuum that will not crack and/or vaporize under the aforesaid mild cracking conditions remains trapped in the mild cracking operation.